Ceramic-metal composites, known as cermets, usually comprise a minor proportion of a metal phase intimately dispersed on a microstructural scale within a major proportion of a ceramic phase. The term ceramic is understood to include oxides, borides, nitrides carbides, silicides and mixtures and combinations thereof such as oxynitrides, typically of metals such as those from Group IIIb (scandium, yttrium, lanthanium, actinium), the lanthanides and actinides, group IVb (titanium, zirconium, hafnium), group Vb (vanadium, niobium, tantalum), group VIb (chromium, molybdenum, tungsten), aluminum and silicon.
Cermets have properties which differ from those of either phase alone. The ceramic provides hardness, abrasion resistance and improves the mechanical properties at high temperatures; the metal improves the strength, ductility, toughness and electrical conductivity. They are conventionally made by well known powder metallurgical methods, i.e. by preparing and mixing individual metal and ceramic powders, pressing into the required shape in a die and subjecting to a sintering heat treatment to bond the particles and develop the required structural integrity, often by direct ceramic-to-ceramic bonding.
Recently, International Patent Applicaton No. PCT/EP 82/00140, publication No. WO 83/00171, has proposed composite materials of aluminum and an aluminum oxycompound, typically alumina, and optionally with additives such as borides, nitrides and carbides, which show great promise for those components of aluminum production cells which in use are normally covered with molten aluminum, including current-carrying components such as a cathode or cathode current feeder, part of a bipolar electrode, an anode current collector for an electrorefining cell, other electrically conducting components such as a cell lining, and non-conductive cell components including separator walls, weirs and packing elements. These composite materials are formed by submitting particles of aluminum and the aluminum oxycompound and/or oxides which will form the aluminum oxycompound by reaction with the aluminum, and optionally with powders of the additives, to a heat treatment. Typically the particles are hot pressed or cold pressed and then heated. However, when a high aluminum content is desired to enhance the electrical conductivity it is difficult by these methods to obtain a structure which remains rigid at the operating temperature (about 1000.degree. C.).
To provide an improved cell component, it was proposed in U.S. patent application Ser. No. 454719 (Sane et al), as yet unpublished, to provide a preformed matrix of alumina, aluminum nitride, aluminum oxynitride, SiAlON, boron nitride, silicon carbide, silicon nitride or aluminum boride which has voids throughout its structure, the voids then being filled with aluminum, e.g. by infiltrating a porous structure with molten aluminum. One of the preferred methods of preforming the matrix was the aluminothermic, carbothermic or combined carbo-aluminothermic reduction of a homogeneous TiO.sub.2.B.sub.2 O.sub.3 oxide glass powder, this method being further described in U.S. patent application Ser. No. 454718 (Sane).
Oxide-boride ceramics and their application as cell components in electrolytic aluminum production cells are also described and claimed in as yet unpublished U.S. patent applications Ser. No. 454671 and Ser. No. 454672, both in the name of de Angelis. In one example, powders of titanium dioxide, boron oxide and aluminum were hot pressed to form a reaction-sintered aluminatitanium diboride composite of uniform grain size (95 vol % of both oxide and boride phases less than or equal to 7 micron, the largest grain size being 10 micron), and 2.6% open porosity. Such reaction-sintered composites are wett-able and resistant to molten aluminum, and are therefore suitable for insert-ion into an aluminum prodction cell for use as a component which may contact the molten aluminum but preferably remains out of contact with molten cryolite.